The present invention relates to an active booster transformer system comprising a -booster transformer core of the type which is applied around an electrical cable for reducing stray currents, the booster transformer core being a body built up of magnetizable material provided with a continuous channel intended to accommodate the cable.
Electric currents generate magnetic fields. In the case of a normal lamp cable with two conductors, one for the current to the lamp and one for the return current, however, the currents in the cable create almost no net field but almost completely cancel out each other""s effect since the conductors are running in parallel adjacent to one another and the currents in the conductors are the same but oppositely directed. If, however, the conductors are separated from one another for some reason, a magnetic field is obtained around each conductor, as in a power line, and if the fitting of the lamp is also earthed via its holder, leakage to earth can arise and the currents are then no longer the same in the conductors, i.e. a net magnetic field is created along the cable even if the conductors are running adjacent to one another. A current which does not follow the intended conductor but takes off on unintended paths, for example via the protective earth, is called a stray current. Due to their electrical equipment and couplings, many houses have existing magnetic fields which interfere with electronic equipment and which can exceed the recommended limit values. The source of the magnetic fields are in most cases stray currents. Stray currents are often due to the fact that, certainly in Sweden, there is an electric system with four conductors in the main line to the building.
FIG. 1 shows a four-conductor system of the known type with a cable K. A current I, which arises in a phase R, if a single-phase load L is connected, has two paths from the transforming station C to earth of the feeding transformer T. The return current can either go via the neutral conductor N with part-current In as intended or via the protective earth and a water pipe V with part-current Iv to earth point of the transformer. When there is current in the water pipe V, this generates a magnetic field around it but also around the cable and return current is jacking so that the magnetic fields from the conductors of the cable no longer neutralize each other. The net current in the cable is as large as Iv. It is often possible to measure up to ten amperes in water, gas and heating pipes in a house which has such installations. When these stray currents continue in lines under the ground outside the building, further magnetic fields are also obtained from these lines.
It is also usual that there are stray currents in existing data networks which, apart from generating magnetic field levels which are potentially harmful to people, can cause serious communication problems. The image on screens is distorted so that it is impossible to work with screens in rooms with magnetic field peaks of over 0.5-1 xcexcT.
The best way to avoid magnetic fields from stray currents is to install a five-conductor system from the beginning. In contrast to the four-conductor system, the protective earth conductor is here separated from the neutral conductor which is why the return current has to take the correct path via the neutral conductor of the electric system back to the transformer. Installation of a five-conductor system leads to low magnetic fields in the house and a low risk of problems with data communication. In hospitals it has long been obligatory to have a five-conductor system. In a new construction, it also involves very little extra cost compared with an installation in an existing house where it can be considerably more costly to change to a five-conductor system since it is not only the electric installations of the house which are affected but also the cable to the substation of the electric power station which must be changed to five conductors.
In smaller buildings like private houses, the stray currents can often be minimized, i.e. stopped by breaking up the alternative current path, e.g. by exchanging a short section of a metallic water, gas or heating pipe for a plastic pipe. This is not a practicable method in larger buildings.
A cost-effective alternative to changing to a five-conductor system is to install at a suitable location, one (or more) booster transformer which is shown diagrammatically in FIG. 2. In principle, FIG. 2 is the same as FIG. 1 with the same designations but with the addition that a booster transformer S applied around a cable K is arranged which is for example but not necessarily of the type which is described in Swedish patent application No. 9900501-9, the complete content of which is herewith incorporated in the present application. The principle of the booster transformer has long been used in electric railways in order to avoid currents from leaking out into the surrounding soil and pipeline systems. The main part of the phase imbalance current I is caused by electromagnetic induction to go through the neutral conductor whereby the stray current Iv is reduced.
A booster transformer of this type can be said to form a single-turn transformer with the four conductors of the cable as windings. The operation of the booster transformer S can be described as the net current (=Iv) in the cable K causing a magnetic flow to be created in the booster transformer S. The flow, in turn, induces a voltage in all conductors of the cable K. The induced voltage counteracts the potential difference between both ends of the conductor PE+N which is caused by its operating current and resistance. This reduces the potential difference which is biasing the stray currents. The efficiency largely depends on the physical dimensions and capability of the material in the core of the booster transformer to conduct magnetic flows.
One of the disadvantages of the technique described above is that it requires a not negligible remaining stray current Iv for forming an effective magnetic flow in the booster transformer. For this reason, it is not possible with this technique to completely eliminate the stray currents but only to reduce them. However, in certain situations it is desirable with a very small remaining stray current. It is of course possible to select a booster transformer with large dimensions but it often becomes unrealistically large and also costly and generally cumbersome.
The above problems have been solved by the booster transformer being included in a system which comprises a current sensor which senses the net current in the cable, an amplifier, the input of which is controlled by the current sensed by the sensor, and a winding located on the booster transformer, the amplifier being arranged to feed the winding on the booster transformer with a current which creates a magnetic flow in the booster transformer, which flow, in turn, induces a longitudinal voltage in the cable which counteracts the stray current. The longitudinal voltage (common-mode voltage) counteracts the stray current without the stray current itself needing to be large enough for creating the necessary magnetic flow. As a result, a booster transformer can be made xe2x80x9cvirtuallyxe2x80x9d many times larger and the stray current outside the cable can be reduced very effectively without the transformer being unwieldy large.
In a special embodiment of the system, the current in the winding is generated by it being connected to the output of an amplifier, the output voltage or output current of which is controlled in such a manner that it is an image of the remaining stray current. In the first case, a net voltage is transformed to the cable which counteracts the voltage which drives the stray current. This is done by an amplifier with low output impedance, i.e. the output voltage is largely independent of the output current. In the second case, the booster transformer is fed with an amplified image of the stray current, with the result that a lower remaining stray current is required for obtaining a sufficiently large magnetic flow in the booster transformer for the stray current to be counteracted. This presupposes that the amplifier has a high output impedance, i.e. the output current is largely independent of the output voltage. An amplifier with moderate output impedance can also be used and the operation of the system is then a combination of the two above.
The current image is obtained by means of a sensor for the stray current. In the first variant of the system, a current sensor, for example of the current clamp type is mounted on the same cable as the booster transformer connected to the amplifier. It is also possible to measure the current on the booster transformer itself which means that there is no need to mount a sensor on the cable, but such a system requires more complicated electronics than the usual sensor. The booster transformer, the electric system, the current sensor and the amplifier together constitute a feedback system which is controlled for the lowest possible stray current.